Metallic mirrors have been widely used for instruments in space and military applications. System performance of the instruments is largely dependent upon the reflective surface of the mirror. Performance of the optical mount, and its thermal and mechanical characteristics also have effects on the performance of the optical component. In actuality the optical mount has a significant impact on performance of the optical system in achieving objectives of any scientific and engineering experiment. When both optical mount and mirror substrate are of the same material there is uniformity of thermal properties. Also the high thermal conductivity of a metal mirror helps decrease cooling time in cryogenic applications.
Many spacecraft systems utilize aluminum materials for structural components in cases of cold or cryogenic use. Aluminum materials may also be used for mirrors as aluminum offers numerous benefits because of its machinability, lightweight, and low cost.
Due to light scattering, which results from poorly polished surfaces, however, bare aluminum cannot be readily implemented as an acceptable mirror material for UV, IR, and visible applications. The scattering lowers the signal-to-noise ratio and throughput.
Existing technology attempts to remedy this dilemma by electro-plating a think layer of electroless nickel to the entire component surface and then optically polishing the plated nickel. The result creates a tradeoff whereby surface roughness is decreased while thermal and mechanical stability of the optic are severely compromised at all but room temperatures. This is especially true for aluminum optics that have been light-weighted. Further complicating matters is the fact that the mount is usually an integrally machined part of an aluminum optic. While these characteristics are great for dimensional requirements and ease of design, they create havoc on the optical performance once all surfaces are evenly plated with nickel. The electroless nickel platings also can cause bi-metallic stresses to deteriorate optical performance. Another problem of plating aluminum with electroless nickel is that manufacturing costs grow because nickel polishes more slowly than conventional optical materials.
One prior technique has overcome such problems yet provides inferior optical performace to one proposed by the present invention. For example, see “Diamonds turn infrared mirrors smooth”, by Daniel Vukobratovich, et al, Optoelectronics World, page S25–S28, October 1998. The prior technique plates an aluminum substrate with an amorphous layer of high-purity aluminum. Then the plated substrate is diamond-turned to produce a mirror with surface roughness of 30 angstroms rms with surface accuracy in terms of surface figure error of 0.380 wave peak-to-valley. This plated substrate is theoretically bimetallic and should experience the bimetallic deformation to some degree. By comparison the present invention provides an aluminum mirror of about 5 angstroms rms surface roughness with surface accuracy in terms of surface figure error as low as one-fifteenth of a wave peak-to-valley without any bimetallic deformations.
In addition to superior optical performance, this invention provides the following advantages by eliminating the electroless nickel plating from the aluminum mirrors:    (1) Drastic cost savings during fabrication.    (2) Reduced risk associated with polishing through nickel to the aluminum. This requires that the part be stripped of the remaining nickel and re-plated. To do so, the optical surface must again be prepared for plating because the stripping procedure etches the aluminum.    (3) Drastic performance improvements. Properly heat-treated bare aluminum performs well in cryogenic conditions without the nickel plating.    (4) Reduced cost of final component characterizations. Plated mirrors that show abnormalities are often tested and re-tested to determine the impact on the system performance. If the problem is identified to be with the nickel plating as is often the case, then the process must be completely repeated by stripping the mirror and starting over.
Properly implemented, therefore, the proposed innovation will eliminate many of the associated problems now common with current aluminum mirror technology, delivering aluminum optics with superior accuracy.